A theory of stochastic quantum dynamics has been developed and used to calculate reorientational correlation times and spin rotational correlation factors for groups undergoing internal rotation in molecules. The theory includes versions that are analogous to the classical Fokker-Planck-Langevin and extended diffusion models, and it allows either Markovian or non-Markovian dynamics. In the diffusion limit, the quantum mechanical theories reduce to their classical analogues when the parameter h/(IK-B-T)1/2 is sufficiently small, where I is the moment of inertia of the internal rotor. In the dilute gas limit, on the other hand, distinctly quantum mechanical effects are apparent regardless of the value of this parameter. These effects arise from tunneling and from echoes in the quantum mechanical correlation functions. The models and their classical analogues are being used to calculate the frequency dependence of the spectral densities in order to assess the effects of internal motion on NOESY spectra and the structural parameters derived from those spectra. The models are also being used to assess the effects of nonMarkovian processes, i.e. processes in which collisions are not instantaneous.